Update Gemfile to use https rubygems url instead of the deprecated one

According to bundler output:

```
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
```
Merge pull request #1 from kovyrin/patch-1

Update Gemfile to use https rubygems url instead of the deprecated one